A dad buys a lie detector robot that slaps a person if they were lying.
Dad: Son, where were you during school hours?
Son: At school.
The robot slaps the son.
Son: Okay I was watching The Secret Life of Pets!
The robot slaps his son again.
Son: Okay I was watching violent movies!
Dad: What?! When I was your age I never watched those kinds of movies!
The robot slaps the dad.
Mum: Haha, after all, he is your son…
The robot slaps the mom.